The invention relates to a curved, ring-type or spiral tubular packing casing, in particular to an artificial sausage casing in the form of a so-called round, comprising unreinforced cellulose and to a method for its production in which viscose (cellulose xanthate) is converted into regenerated cellulose. The helically wound tube obtained after drying can be divided into individual curved or annular sections which, when used as artificial sausage casing, are filled with sausage meat and further processed.
Packing casings of this type have been known for a long time. They are produced by the viscose method in which the alkaline viscose solution is extruded through an annular slot die directly into an acid coagulation bath. The coagulated tube is converted into cellulose hydrate gel by treatment with acid regeneration liquid and is thereafter dried in the inflated state.
In order to produce the permanently curved tube form by the viscose method, the coagulated and regenerated tube of cellulose hydrate gel is dried in the inflated and spirally wound state. Thus, DE-C-667,016 (=US-A-2,136,566) describes a drying process in which the tube, spirally wound on a heated drum and inflated with air, is dried in two stages. First, the inner side of the tube in contact with the drum is dried, and thereafter the tube is dried on all sides with air until completely dry. Because of the one-sided drying, the inner side bearing on the drum shrinks to a greater extent than the outer side which is dried later. Due to the cellulose hardening in the heat, the tube assumes the permanently spiral shape. This method requires a high degree of servicing and is suitable only for non-continuous operation. The unfavorable material properties of the tube casing obtained are a further disadvantage.
In a further development the spiral form is produced on the coagulated and not yet completely regenerated tube. In this method, described in DE-C-932,713 (=US-A-2,925,621), the air-inflated, coagulated tube is passed helically over a drum-shaped support through a regeneration bath. The regenerated cellulose tube running off the support is permanently shaped. DE-C-1,163,701 (=US-A-3,155,752) describes a cylindrical support suitable for carrying out this method. According to the information in DE-C-932,713 the cellulose tube casings regenerated in spiral winding should have better dimensional stability and smaller ring diameters compared to those prepared according to DE-C-667,016 which obtain their spiral shape only by the one-sided shrinkage upon drying. However, the method of DE-C-932,713 has the disadvantage that production interruptions are necessary for removal of the reaction gases arising in the tube upon regeneration of the cellulose. The tube casings obtained show widely differing properties, in particular as regards bursting pressure, caliber (casing diameter), ring diameter, expansion behavior and strength. The strength of the casings is, moreover, inadequate. It does not permit the casings to be provided in the watered state with a metal clip. In order to prevent damage to these casings during tying-off, a special thread is required. When used as sausage casing for boiling sausage, undesired wrinkle formation and jelly deposition occur after only a short storage time. Wrinkled sausage casings are not only unattractive, but the wrinkled areas also prevent penetration of smoke upon smoking of the sausage.
It is furthermore known in the prior art to produce curved sausages with tube casings of thermoplastic material. Thus, according to DE-C-2,163,764 (=US-A-3,950,469) the molten plastic (polyethylene terephthalate) is extruded through a tubular die, with an eccentrically designed ring slot, to give a tube which, corresponding to the shape of the ring slot, has a non-uniform thickness over the periphery. The extruded tube is stretched biaxially, and subsequently further stretched monoaxially in the longitudinal direction. The synthetic tube obtained is not yet curved. Only when a sausage mixture is pressed in should the straight synthetic tube be given a curved form by the action of the filling pressure. The thin-walled half of the tube is expanded radially more easily than the thick-walled half of the tube. The result of this is that the tube is radially expanded predominantly in the area of the thin-walled half of the tube, which occurs at the expense of a partial reduction in the length of the tube. This reduction in the length of the thin-walled part of the tube brings about the curvature of the tube when the sausage mixture is pressed in.
DE-C-2,314,767 (=US-A-4,081,562) has disclosed a method for the production of a curved sausage casing of edible materials, in particular animal skin fiber compositions. In the example collagen is extruded through a tubular die with an eccentrically mounted pellet. By this means a tube casing is produced which is of non-uniform thickness over the periphery and which, as a result of the nonuniform width of the die gap, curves on emerging from the die. It is then filled with blown air, led off helically in uniform turns and dried, the thicker areas at all times being subjected to the greatest expansion. Sausage casings of this material are not suitable for a number of types of sausage because of the lack of transparency. Moreover, in order to increase their strength, these casings must be watered in common salt solutions. This leads to pollution of the waste water and to corrosion of the processing machines.